1. Field of the Invention
The present invention relates to an apparatus which allows surfaces to be perfused with a soluble biologically active chemical agent at controllable flux rates, and a method for determining the effects therefrom; particularly, but not exclusively, biocidal agents meant to be incorporated in antifouling coatings and coverings.
2. Description of the Prior Art
Improvements in ship hull design over the past years have been aimed at maximizing vessel speed and range by minimizing hydrohynamic drag. In order to take full advantage of these improvements the hull must remain smooth, that is, clean and free from fouling organisms. Marine fouling increases hull roughness and therefore drag which leads to inncreased fuel consumption and decreased overall efficiency of the vessel.
The accumulation of fouling organisms on buoys, pier pilings, discharge pumps, ship hulls, propeller blades, hydrofoil legs, and the like greatly increases maintenance costs, and may adversely affect function which may constitute a source of danger, particularly when affected sites are necessary for normal operation. In the case of a sonar dome, which ideally has a smooth rubber surface, a layer of fouling organisms tends to deflect and scatter the sonar beams.
The prior art illustrates a number of combinations of paint and polymeric materials containing biocidal agents which prevent accumulation of fouling organisms. For example, antifouling coverings using rubber which contains organometal toxicants as a reservoir layer onto which is laminated an elastomertic layer that serves as a toxic transfer control sheet are exemplified in U.S. Pat. Nos. 3,426,473, 3,497,990, 3,505,758, and 4,401,703. The most prevalent biocidal agents used in modern antifouling coatings are copper and organotin compounds.
Antifouling coatings are designed to release biocides over several years at a controlled rate. The biocides are released from their binder materials by film erosion, ablation, and/or diffusion and prevent attachment of fouling organisms or kill them before they grow to significant size. Various polymeric binder compounds have been developed in an effort to control the release rates of these toxicants as exemplified, by way of example, in U.S. Pat. Nos. 3,016,369, 3,382,264, 3,930,971, 3,979,354, 4,064,338, 4,075,319, 4,174,339, 4,389,460, and 4,480,056. However, many of these older diffusion-dependent coatings exhibit a characteristic decrease in toxicant release rate where initial release rates are very high but decrease with time. After decreasing, this rate must be sufficient to control the target marine growth. Therefore, formulations designed for long life may have unacceptably high initial release rates.
Studies conducted by the Environmental Protection Agency (EPA), the U.S. Navy, and others have indicated that concentrations of organotin currently in the waters of the United States may pose unreasonable risks to oysters, clams, fish, and other aquatic life forms. There is increasing evidence to suggest that the excessive release of powerful biocides, such as tributyltin (TBT), from antifouling coatings may cause environmental damage. While coatings based on organotin compounds have proven more effective in inhibiting marine growth than the conventional copper-based coatings, the organotin biocide can affect non-target marine and freshwater organisms. This is of particular concern when many ships are berthed near areas which contain economically significant organisms. For example, TBT is toxic to Pacific oysters (Crassostrea gigas) at very low levels; slow growth and high mortality occur at concentrations as low as 0.05 .mu.g/L.
In an effort to protect the aquatic environment, Great Britain, France, the United States, and others have imposed legislative restrictions on the use of organotin paints. The U.S. Congress has passed the Organotin Antifouling Paint Control Act of 1987 prohibiting the use of organotin antifouling paints on vessels less than 25 meters in length. This Act requires the Administrator of the EPA to certify each antifouling paint containing organotin to be at release rates of not more than 4.0 micrograms per square centimeter per day, as determined by the Americal Society for Testing Materials (ASTM) standard test method which the EPA required in its July 29, 1986 data call-in notice or by any similar test method specified by the Administrator. The Act also instructed the EPA Administrator and the Secretary of the Navy to conduct research into chemical and non-chemical alternatives to antifouling paints containing organotin. Under these circumstances it is highly desirable to develop effective antifoulant coatings with lower, more precisely controlled release rates.
Paint manufacturers have therefore improved coating formulations by developing organometallic polymer coating systems based upon organometal acrylate polymers which demonstrate a constant delivery rate of the toxicant. Binding TBT to a polymer is effective in regulating the release rate of the biocide from the organometallic polymer coating. Since toxicant release in these systems depends on a hydrolysis and lixiviation process, some formulators have concentrated on varying the polymer characteristics in order to control the erosion rate, as evidenced, for example, by U.S. Pat. Nos. 4,075,319 and 4,082,709. Others, however, have included hydrophobic additives such as chlorinated rubbers, polyacrylate esters, and silicones to retard and control release, as evidenced, for example, by U.S. Pat. No. 4,021,392. In addition, alternatives to organometals, such as the natural compounds from whip corals that appear to prevent biological attachment, are currently being isolated by researchers at Duke University and the University of Delaware.
However, as constitutent coating materials are developed which tend to produce steady state release rates, little scientific data exists as to what minimum release rate will be effective for control of target organisms. Although it is not precisely known how rapidly organotin compounds, such as TBT, must be released in order to be effective against fouling organisms, there is evidence that current antifouling paint release rates are several times higher than necessary to prevent significant fouling. It would therefore be useful to determine the minimum release rate of organometallic and other biocides in order to reduce environmental damage without decreasing the effectiveness of antifouling coatings.
Minimizing the threat to non-target organisms requires controlling and minimizing the biocide release rate to only that level which is sufficient to prevent attachment. Determining optimum release rates by current methods requires that an array of paint formulations be developed and tested for their release rate and antifouling effectiveness. Formulating and testing new antifouling paints to a minimum or optimum release rate would be desirable, but is a difficult, expensive, and time-consuming process. In this field, it would be useful to have an estimate of a toxicant's minimum effective release rate before the formulation process begins. That is, the ability to determine an optimum biocide release rate under natural conditions would greatly facilitate the development of effective antifouling coatings whose environmental impacts are negligible.